Biomedical Engineering Reference
In-Depth Information
on a macroscopic level within the finished structure” [64]. Thus,
composites are always heterogeneous. Furthermore, the phases
of any composite retain their identities and properties, and are
bonded, which is why an interface is maintained between them. This
provides improved specific or synergistic characteristics that are
not obtainable by any of the original phases alone [65]. Following
the point of view of some predecessors, we also consider that “for
the purpose of this review, composites are defined as those having a
distinct phase distributed through their bulk, as opposed to modular
or coated components” [66, p. 1329]. For this reason, with a few
important exceptions, the structures obtained by soaking of various
materials in supersaturated solutions containing ions of calcium and
orthophosphate (e.g., Refs. [67-73]), those obtained by coating of
various materials by calcium orthophosphates (e.g., Refs. [74-82]),
as well as calcium orthophosphates coated by other compounds
[83-87] have not been considered; however, composite coatings
have been considered. Occasionally, porous calcium orthophosphate
scaffolds filled by cells inside the pores [88-91], as well as calcium
orthophosphates impregnated by biologically active substances [92,
93] are also defined as composites and/or hybrids; nevertheless,
such structures have not been considered in this review either.
In any composite, there are two major categories of constituent
materials: a matrix (or a continuous phase) and (a) dispersed
phase(s). To create a composite, at least one portion of each type
is required. General information on the major fabrication and
processing techniques might be found elsewhere [66, 94]. The
continuous phase is responsible for filling the volume, as well as it
surrounds and supports the dispersed material(s) by maintaining
their relative positions. The dispersed phase(s) is(are) usually
responsible for enhancing one or more properties of the matrix.
Most of the composites target an enhancement of mechanical
properties of the matrix, such as stiffness and strength; however,
other properties, such as erosion stability, transport properties
(electrical or thermal), radiopacity, density or biocompatibility
might also be of a great interest. This synergism produces the
properties, which are unavailable from the individual constituent
materials [94, 95]. What's more, by controlling the volume fractions
and local and global arrangement of the dispersed phase, the
properties and design of composites can be varied and tailored to
suit the necessary conditions. For example, in the case of ceramics,
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